1. Field of the Invention
The present disclosure relates generally to light-emitting devices and more particularly to packages for uniformly dispersing light from light-emitting diodes.
2. Description of the Prior Art
A light-emitting device includes a light source and a package for supporting. the light source and directing, focusing, filtering, or enhancing light emitted from the light source. Some examples of light sources include a light-emitting diode (LED), an incandescent lamp, a sapphire crystal light, and a fluorescent lamp.
An LED is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This effect is a form of electroluminescence. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible or near-ultraviolet. Advantages of LEDs over other lighting sources include compactness, very low weight, low power consumption, simple and inexpensive manufacturing, freedom from burn-out problems, high vibration resistance, and an ability to endure frequent repetitive operations. In addition to having widespread applications for electronic products such as indicator lights and so forth, LEDs also have become an important alternative light source for various applications where incandescent and fluorescent lamps have traditionally predominated.
While LEDs are generally monochromatic, LEDs can also be used produce white light, for example, using phosphors as light “converters.” In a typical LED-based white light producing device, an LED that produces a monochromatic visible light is encapsulated in a material containing a compensatory phosphor. The wavelength of the light emitted from the compensatory phosphor is complementary to the wavelength of the light emitted by the LED such that the wavelengths from the LED and the compensatory phosphor mix together to produce white light. For instance, a blue LED-based white light source produces white light by using a blue monochromatic LED and a phosphor that emits a complementary yellow hue when excited by the blue light. In these devices the amount of the phosphor in the encapsulant is carefully controlled such that a fraction of the blue light is absorbed by the phosphor while the remainder passes unabsorbed. The complementary yellow hue of the light emitted by the phosphor and the unabsorbed blue light mix to produce white light.
In another typical LED-based white light producing device, multiple monochromatic LED elements are encapsulated in a transparent material. For example, a red LED element, two green LED elements and a blue LED element can form a red-green-green-blue (RGGB) LED light source. Current can be applied independently to each of the LED elements to adjust the color balance. Thus, a white light can be produced.
Unfortunately, the color balance of the white light can vary depending on an angle from which the light is viewed, which results in a non-uniform color distribution. Attempts have been made using special-mixing lenses to compensate for the non-uniformity of the color distribution. However, while the variation may be reduced, the color still varies noticeably depending on the angle of the emitted illumination, or the angle from which the illumination is received or viewed. Such color non-uniformity can negatively affect designs for light sources such as spot lights and other general lighting applications, and color display technologies such as active matrix thin film transistor liquid crystal displays (TFTLCDs) in applications such as consumer computer and television monitors, projection TVs, large advertising displays. One solution to the problem of color variation is to use a secondary lens with a light mixing design on the light emitting device. Unfortunately, the secondary lens generally causes a 40% to 50% reduction in light intensity output by the light emitting device.
The quality of color, also known as color rendition, is also very important in many applications. For example, medical personnel rely on color for identifying tissues during surgery. One measure of color rendition is the ability of a light source to reproduce the colors of various objects being lit by the source, which can be quantified by a color rendering index (CRI). The best possible rendition of colors is specified by a CRI of 100, while the poorest rendition is specified by a CRI of 0. In applications such as surgery, a CRI of less than 70 results in a drop out of many colors and provides poor illumination, making tissue identification difficult. Typically, a CRI of greater than 80-90 is preferred for medical applications. The CRI of an incandescent light bulb, which emits essentially black body radiation, is nearly one hundred. However, an incandescent lamp also produces a lot of heat. What is needed is a cool light source with a CRI greater than 80-90. While a white light LED source can be adjusted to emit light having a high CRI in one particular direction, what is needed is an LED source that can emit white light having a high CRI uniformly over a wide range of angles.
Given the importance of LEDs as light sources, particularly LEDs using multiple color elements, there is a need for improved LED packaging methods and materials to alleviate the above-identified problems. There is a further need for methods and materials that can also reduce light lost at large angles and allow LEDs to produce higher optical performance (Lumens/package) from a smaller package or footprint (Lumens/area), which are critical for many light source applications.